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This journal is © The Royal Society of Chemistry 2016 Chem. Commun., 2016, 52, 4659--4662 | 4659 Cite this: Chem. Commun., 2016, 52, 4659 An acido- and photochromic molecular device that mimics triode actionP. Remo ´ n, a S. M. Li, a M. Grøtli, b U. Pischel* c and J. Andre ´ asson* a The photo-controlled shift of pH titration curves, describing the acidochromic behaviour of a spiropyran switch network, was harnessed for the realisation of a molecular triode. The intricate network can be correctly interpreted with respect to the pH dependence of the main involved species. The fascinating opportunities offered by light-controlled molecular switches are currently unfolding in research areas such as chemical biology, nanochemistry, materials science, and molecular information processing. 1–9 This preference is triggered by the possibility of changing molecular and system’s properties by light exposure in a reversible, waste-free, non-invasive, and spatiotemporally controlled fashion. With a view on this, light has been increasingly used as a stimulus to devise molecular devices capable of performing logic operations 5,10–14 with different degrees of complexity, e.g., simple logic gates (AND, OR, NOR, etc.), elementary arithmetic operations (addition, subtraction), logic circuits (multiplexer, encoder, etc.), and memory devices. 2,15–22 In the semiconductor-driven world triodes/transistors play an archetypal role. Inspired by this, it is very appealing to devise molecules that are capable of mimicking the performance of these analogue electronic building blocks. Despite the conven- tional use of electrical signalling, chemical and photophysical phenomena can be interpreted along the lines of a molecular triode or transistor as well. One strategy is the exploitation of photochromic (dithienylethene) supramolecular systems for the all-photonic realization of photo-controlled emission modulation via energy transfer. 23,24 A different approach was followed using a pH-sensitive fluorophore, whose electron-transfer-induced emission quenching can be additionally controlled by metal cation complexation. The resulting pK a shifts resemble the characteristics of triode iV curves. 25 Finally, very recently the peculiar temperature-dependent operation of a fluorescent electro- chemical switch was harnessed for demonstrating a transistor function. 26 Some of these systems require either intensive synthetic labour or a very specific chemical design. Others work with metal-cation inputs which may cause problems for the recycling of the function. In an effort to base our chemical design on the generally and widely used class of photofunctional spiropyrans we came across a so far overlooked photo-controlled hysteresis effect in the pH titration curves describing the integrated acidochromic behaviour of such systems. Their application in bio-relevant contexts such as light-controlled DNA binding 27–29 and inter- actions on proteins, 30 bioimaging, 31–33 or biosensing 34 provides an additional incentive to explore their inherent functional character beyond the photochromic switching itself, for example for devising a molecular triode. This complements the conceptual importance of spiro-pyrans/-oxazines in molecular information processing as exemplified by molecular memories and Fuzzy logics. 35–39 Spiropyrans, such as the water-soluble 1, are implied in photo- and acidochromic switching between three states: spiro form (SP), merocyanine (MC), and protonated merocyanine (MCH); Scheme 1. 40 In a very acidic solution (pH o 0.5) the protonated spiro form SPH is abundant, while the undesired hydrolysis of the MC form intervenes especially at basic pH (pH 4 8). 40,41 Therefore we limited our study to an intermediate pH window between 1.5 and 8. In aqueous solution at pH Z 6 1SP is in thermal (‘‘dark’’) equilibrium with 1MC (K = k 1 /k 2 = 0.16 at pH 7) which can be displaced by protonation of the merocyanine to yield the thermally stable 1MCH (pK a I = 4.4). 40 Hence, at high pH (pH Z 6) the SP form is dominant, whereas at low pH (pH r 4.5) a Department of Chemistry and Chemical Engineering, Physical Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden. E-mail: [email protected] b Department of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Gothenburg, Sweden c CIQSO - Centre for Research in Sustainable Chemistry and Department of Chemistry, Campus El Carmen, University of Huelva, E-21071 Huelva, Spain. E-mail: [email protected] Electronic supplementary information (ESI) available: Additional experimental data, procedures for obtaining the concentrations of the different involved open and closed forms of 1 at each pH for the thermal and photochromic processes, and modelling of the network. See DOI: 10.1039/c6cc00840b Received 27th January 2016, Accepted 29th February 2016 DOI: 10.1039/c6cc00840b www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 07 March 2016. Downloaded by Chalmers Tekniska Hogskola on 17/03/2016 11:03:29. View Article Online View Journal | View Issue

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Page 1: An acido- and photochromic molecular device that mimics ...publications.lib.chalmers.se/records/fulltext/234475/local_234475.pdfAn acido- and photochromic molecular device that mimics

This journal is©The Royal Society of Chemistry 2016 Chem. Commun., 2016, 52, 4659--4662 | 4659

Cite this:Chem. Commun., 2016,

52, 4659

An acido- and photochromic molecular devicethat mimics triode action†

P. Remon,a S. M. Li,a M. Grøtli,b U. Pischel*c and J. Andreasson*a

The photo-controlled shift of pH titration curves, describing the

acidochromic behaviour of a spiropyran switch network, was harnessed

for the realisation of a molecular triode. The intricate network can be

correctly interpreted with respect to the pH dependence of the main

involved species.

The fascinating opportunities offered by light-controlled molecularswitches are currently unfolding in research areas such aschemical biology, nanochemistry, materials science, and molecularinformation processing.1–9 This preference is triggered by thepossibility of changing molecular and system’s properties bylight exposure in a reversible, waste-free, non-invasive, andspatiotemporally controlled fashion. With a view on this, lighthas been increasingly used as a stimulus to devise moleculardevices capable of performing logic operations5,10–14 with differentdegrees of complexity, e.g., simple logic gates (AND, OR, NOR, etc.),elementary arithmetic operations (addition, subtraction), logiccircuits (multiplexer, encoder, etc.), and memory devices.2,15–22

In the semiconductor-driven world triodes/transistors playan archetypal role. Inspired by this, it is very appealing to devisemolecules that are capable of mimicking the performance ofthese analogue electronic building blocks. Despite the conven-tional use of electrical signalling, chemical and photophysicalphenomena can be interpreted along the lines of a moleculartriode or transistor as well. One strategy is the exploitation ofphotochromic (dithienylethene) supramolecular systems for

the all-photonic realization of photo-controlled emission modulationvia energy transfer.23,24 A different approach was followed usinga pH-sensitive fluorophore, whose electron-transfer-inducedemission quenching can be additionally controlled by metalcation complexation. The resulting pKa shifts resemble thecharacteristics of triode i–V curves.25 Finally, very recently thepeculiar temperature-dependent operation of a fluorescent electro-chemical switch was harnessed for demonstrating a transistorfunction.26 Some of these systems require either intensive syntheticlabour or a very specific chemical design. Others work withmetal-cation inputs which may cause problems for the recyclingof the function.

In an effort to base our chemical design on the generally andwidely used class of photofunctional spiropyrans we cameacross a so far overlooked photo-controlled hysteresis effectin the pH titration curves describing the integrated acidochromicbehaviour of such systems. Their application in bio-relevantcontexts such as light-controlled DNA binding27–29 and inter-actions on proteins,30 bioimaging,31–33 or biosensing34 providesan additional incentive to explore their inherent functionalcharacter beyond the photochromic switching itself, for examplefor devising a molecular triode. This complements the conceptualimportance of spiro-pyrans/-oxazines in molecular informationprocessing as exemplified by molecular memories and Fuzzylogics.35–39

Spiropyrans, such as the water-soluble 1, are implied in photo-and acidochromic switching between three states: spiro form(SP), merocyanine (MC), and protonated merocyanine (MCH);Scheme 1.40 In a very acidic solution (pH o 0.5) the protonatedspiro form SPH is abundant, while the undesired hydrolysis ofthe MC form intervenes especially at basic pH (pH 4 8).40,41

Therefore we limited our study to an intermediate pH windowbetween 1.5 and 8.

In aqueous solution at pH Z 6 1SP is in thermal (‘‘dark’’)equilibrium with 1MC (K = k1/k2 = 0.16 at pH 7) which can bedisplaced by protonation of the merocyanine to yield thethermally stable 1MCH (pKa

I = 4.4).40 Hence, at high pH(pH Z 6) the SP form is dominant, whereas at low pH (pH r 4.5)

a Department of Chemistry and Chemical Engineering, Physical Chemistry,

Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.

E-mail: [email protected] Department of Chemistry and Molecular Biology, University of Gothenburg,

SE-412 96 Gothenburg, Swedenc CIQSO - Centre for Research in Sustainable Chemistry and Department of

Chemistry, Campus El Carmen, University of Huelva, E-21071 Huelva, Spain.

E-mail: [email protected]

† Electronic supplementary information (ESI) available: Additional experimentaldata, procedures for obtaining the concentrations of the different involved openand closed forms of 1 at each pH for the thermal and photochromic processes,and modelling of the network. See DOI: 10.1039/c6cc00840b

Received 27th January 2016,Accepted 29th February 2016

DOI: 10.1039/c6cc00840b

www.rsc.org/chemcomm

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4660 | Chem. Commun., 2016, 52, 4659--4662 This journal is©The Royal Society of Chemistry 2016

the MCH form prevails. This is clearly manifested in thecorresponding UV/vis absorption spectra (see Fig. 1). The small1MC absorption band (lmax = 515 nm) at pH 6 is contrasted bythe dominating 1MCH absorption band (lmax = 416 nm) at pH 3.

The situation changes dramatically when UV light (l = 254 nm,power density of 700 mW cm�2) is applied until reaching thephotostationary state (PSS). Now 1SP is photoisomerised to 1MC atpH Z 6. The quantum yields (F) of the ring-opening and closingpaths are 0.040 and 0.012, respectively.‡ 40 However, at pH r 4.5UV exposure culminates in the enrichment of the sample in 1SP;see Fig. 1. This is reasoned with the highly competitive ring closureof the 1MCH form to 1SP (F = 0.21).

The photo-driven variation of the composition of the switchnetwork has consequences for the titration curve monitoringthe normalised concentration of the merocyanine form ([1MC]/[1MC]max) depending on the pH; see Fig. 2. 1MC can be convenientlyfollowed by its absorbance at l = 515 nm, where none of theother forms of 1 absorb. In the absence of light irradiation, theconcentration of 1MC is mainly determined by the degree ofprotonation producing 1MCH. However, as described above,

1MCH is efficiently photo-converted to 1SP and thereby removedfrom the protonation equilibrium involving 1MC. Consequently,the re-establishment of the thus disturbed equilibrium leads to adecreased relative 1MC concentration as compared to the same pHin the absence of UV irradiation. This is experimentally expressedin a positive shift of the ‘‘apparent pKa’’ value by about 1.6 units.Note that this shift is the result of the integrated behaviour of theswitch network and conceptually different from the observationof pKa shifts on photo-conversion between two isomers withdiffering basicity.42,43

Furthermore it was foreseen that the irradiation with anattenuated UV intensity would reveal an intermediate situation,signalled by a pH titration curve situated between the twobeforehand discussed cases (thermal processes and irradiationat full UV intensity). Gratifyingly this was indeed verified for anattenuation to 1.3% of full UV intensity; see Fig. 2.

The working principle and characteristics of a triode can bedescribed as follows. Upon heating the cathode by a filament athermoionic emission of electrons results, which is collected bya positively charged anode plate. On their way the electronshave to pass through a grid which acts like a gate. The anodecurrent–voltage (iA–VA) curve can be shifted by the variation ofthe grid voltage (VG), resulting in the situation shown in Fig. 3.The comparison of these electronic characteristics with theherein observed shift of the pH titration curve allows thedrawing of analogies. In our molecular version the triodeproduces an output in the form of 1MC concentration, whichis varied by pH and UV intensity, corresponding to iA, VA, andVG, respectively.

Based on the relevant rate- and protonation constants, thepH titration curve monitoring the concentration of 1MC wasmodelled and it showed virtually quantitative agreement withthe experimental data (see ESI†). The discussed intricate interplaybetween the different forms of the network was further validatedby monitoring the 1SP and 1MCH forms. Their observation is notdirectly linked to the readout of the absorbance at a specific

Scheme 1 Switch network of spiropyran 1. The associated pH values areapproximate and included for the purpose of orientation. The hereinrelevant rate constants of the thermally-induced processes (red) andphotochromic processes (black) are colour-coded; see also ESI†.

Fig. 1 UV/vis absorption spectra of 1 (20 mM in water) for the thermalequilibrium (pH 3, black full line; pH 6, red full line) and in the PSS (pH 3,black dashed line; pH 6, red dashed line).

Fig. 2 pH titration curves monitoring the normalised 1MC concentration([1MC]/[1MC]max). The experimental data points were obtained for fullyequilibrated systems (i.e. thermal equilibrium and/or PSS). Colour codes:red = absence of irradiation (dark reaction); black = irradiation with 254 nmlight; blue = irradiation with 254 nm light with an attenuated UV intensity(to 1.3%). The lines correspond to sigmoid fits for guiding the eye.

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wavelength as their absorption spectra overlap significantly.Therefore a system of equations that allows the numericalcalculation of their concentrations was developed (see ESI†).Expectedly and in accordance with the corresponding modelling,monitoring the normalised concentration of 1MCH revealedopposite pH dependence compared to that established for 1MC,see ESI.† Finally, also the plot of the normalised concentration of1SP confirmed the applied lines of thought. Interestingly, but notunexpectedly, the titration curves for 1SP are not shifted butinverted (Fig. 4). In the absence of irradiation, the SP concentrationis elevated at high pH (the 1SP/1MC equilibrium is on the side ofthe ring-closed SP form; see above) and diminished at low pH(mostly due to the shifted equilibrium caused by the formation ofthe thermally stable 1MCH). However, under irradiation this isinverted: the relative concentration of 1SP is high at low pH (due tothe efficient 1MCH - 1SP conversion) and low at high pH (dueto the 1SP - 1MC conversion). Curiously, the application ofattenuated UV intensity balances the network in such a way thatvirtually no pH dependence of the relative 1SP concentration isobserved.

In conclusion, the herein discovered photo-controlled shiftof the pH titration curve, describing the acidochromic behaviourof the water-soluble spiropyran 1, can be harnessed for the designof a molecular triode. The output of this device is read through theexclusive absorption signal of the MC form at 515 nm and the gridvoltage is presented by the UV intensity. The described effects openthe door to integrate the triode function with other features thatcan be introduced for example on the synthetically flexible indolenitrogen position.

Financial support from the Spanish Ministerio de Economıa yCompetitividad (grants CTQ2011-28390 and CTQ2014-54729-C2-1-P for U. P.), the Junta de Andalucıa (P12-FQM-2140), FEDER,and the Swedish Research Council VR (grant 622-2010-280 forJ. A.) is gratefully acknowledged.

Notes and references‡ It was assumed that the isomerisation quantum yields are independentof the irradiation wavelength.

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Fig. 3 Characteristic i–V curves of a triode upon variation of the gridvoltage (VG). The inset shows the electronic representation of a triode andthe assignment of the applied potentials.

Fig. 4 pH titration curves monitoring the normalised 1SP concentration([1SP]/[1SP]max) in the dark (red), under full UV irradiation (254 nm; black) orattenuated UV irradiation (to 1.3% of full intensity; blue). The data wereobtained under equilibrium conditions (i.e., thermal equilibrium and/orPSS). The sigmoid curves are meant to guide the eye.

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